Voltage References

CIRCUIT DESCRIPTION
The MC3357 is a low power FM IF circuit designed
primarily for use in voice communication scanning
receivers.
The mixer-oscillator combination converts the input
frequency (e.g., 10.7 MHz) down to 455 kHz, where, after
external bandpass filtering, most of the amplification
is done. The audio is recovered using a conventional
quadrature FM detector. The absence of an input signal
is indicated by the presence of noise above the desired
audio frequencies. This "noise band" is monitored by
an active filter and a detector. A squelch trigger circuit
indicates the presence of noise (or a tone) by an output
which can be used to control scanning. At the same
time, an internal switch is operated which can be used
to mute the audio.
The oscillator is an internally-biased Colpitts type with
the collector, base, and emitter connections at Pins 4,
1, and 2 respectively. A crystal can be used in place of
the usual coil.
The mixer is doubly-balanced to reduce spurious responses.
The input impedance at Pin 16 is set by a 3.0
kG internal biasing resistor and has low capacitance,
allowing the circuit to be preceded by a crystal filter.
The collector output at Pin 3 must be dc connected to
B +, below which it can swing 0.5 V.
After suitable bandpass filtering (ceramic or LC) the
signal goes to the input of a five-stage limiter at Pin 5.
The output of the limiter at Pin 7 drives a multiplier,
MC3357
MOTOROLA LINEAR/INTERFACE ICs DEVICE DATA
8-67
both internally directly, and externally through a quadrature
coil, to detect the FM. The output at Pin 7 is also
used to supply dc feedback to Pin 5. The other side of
the first limiter stage is decoupled at Pin 6.
The recovered audio is partially filtered, then buffered
giving an impedance of around 400 G at Pin 9. The
signal still requires de-emphasis, volume control and
further amplification before driving a loudspeaker.
A simple inverting op amp is provided with an output
at Pin 11 providing dc bias (externally) to the input at
Pin 10 which is referred internally to 2.0 V. A filter can
be made with external impedance elements to discriminate
between frequencies. With an external AM detector
the filtered audio signal can be checked for the presence
of noise above the normal audio band, or a tone
signal. This information is applied to Pin 12.
An external positive bias to Pin 12 sets up the squelch
trigger circuit such that Pin 13 is low at an impedance
level of around 60 kG, and the audio mute (Pin 14) is
open circuit. If Pin 12 is pulled down to 0.7 V by the
noise or tone detector, Pin 13 will rise to approximately
0.5 Vdc below supply where it can support a load current
of around 500 pA and Pin 14 is internally short-circuited
to ground. There is 100 mV of hysteresis at Pin 12 to
prevent jitter. Audio muting is accomplished by connecting
Pin 14 to a high-impedance ground-reference
point in the audio path between Pin 9 and the audio
amplifier.